Prismatic Eyepiece Attachment for Telescopic Gun Sights

ABSTRACT

An attachment device that shifts the exit pupil position of a telescopic gun sight is introduced. The attachment includes a refracting wedge prism to tilt the viewing axis of the telescopic sight. This, in turn, shifts the position of the exit pupil of the telescopic sight and makes it possible to view the target image from an eye position offset from the sight&#39;s optical axis. The invention provides improved ergonomics for the shooter.

I. CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of my co-pending application Ser. No. 14,732,524, filed on Jun. 5, 2015. A new figure (FIG. 4) has been added to better illustrate the invention in perspective view. All the features of the invention described or shown in the present application have been previously disclosed in the specifications or the drawings of the parent application. No new matter has been added.

II. FIELD OF THE INVENTION

This application is in the field of telescopic gun sights used on small arms. Specifically, it describes an attachment which can be connected to the eyepiece side of a telescopic gun sight to move its eyepoint to a more comfortable position. The shooter can maintain a proper cheek weld on the weapon stock, as taught in small weapon training for achieving high accuracy of target hits.

III. BACKGROUND OF THE INVENTION

Telescopic gun sights also called riflescopes are widely used to assist aiming weapons, such as rifles, by eye. To use a telescopic sight, the shooter must look through the sight's eyepiece such that his eye pupil is aligned with the sights's optical axis. In addition, to see the full field of view, he has to position his eye at a hypothetical pupil known as the “exit pupil” which is positioned at set distance (about 4 inches) behind the eyepiece. This distance is called “eye relief”. The concepts of exit pupil and eye relief are well-known in the art and are described in most riflescope manufacturers' catalogs.

Most all telescopic sights are mounted over the receiver of small caliber weapons and above the iron sights as illustrated in FIG. 1(a). The large diameter of the telescopic sights objective lens forces the shooter to raise his head to an unnatural position above his cheek weld, i.e. above where the shooters cheek would normally be against the weapon stock while viewing along the line-of-sight of the iron sights. The required shift in head position is more pronounced when the riflescope has a large objective lens which necessitates a high mount. For instance, a riflescope with a 56 mm objective lens requires at least 30 mm clearance between its optical axis and the rifle's receiver top surface. A high riflescope mount is also required on certain bolt-action rifles (e.g. the famous Mauser M-98) to allow for the bolt to operate freely.

To help the shooter align his eye with the optical axis of a high-mounted riflescope, some rifle stocks are equipped with a raised or adjustable comb (see FIG. 1(a)). However, a raised comb spoils the natural lines of the rifle stock. A raised comb also make the rifle unsuitable for use with open sights.

Another situation where an optical sight with an offset eyepoint becomes desirable is when a secondary sight (often a non-magnifying reflector sight) is mounted on the top of the main telescopic sight (See FIG. 1(b)). This configuration is very useful in combat situations where a soldier needs to engage enemy positions at both close and far distances in quick succession. A soldier can use the non-magnifying reflector sight for engaging targets at close distances. Reflector sights have wide field of view and no pre-set eye relief. This makes them ideal for aiming at close-range and/or moving targets. The telescopic sight is used to aim at distant targets. A major drawback of the configuration shown in FIG. 1(b) is that the telescopic sight and the reflector sight require two separate eye positions.

The present invention introduces an attachment device which can be connected to the eyepiece end of a telescopic sight to move the position of its exit pupil. By moving the exit pupil to a lower position, the shooter can maintain a proper cheek weld on the weapon stock and achieve high aiming accuracy.

IV. SUMMARY OF THE INVENTION

In accordance with a preferred embodiment, the attachment comprises a wedge prism mounted in a cylindrical housing. The housing can be attached snugly to the eyepiece lens of a telescopic gun sight. The refractive power of the prism causes the sight's eyepoint to shift to an offset position away from its main optical axis. The offset position of the eyepoint allows for a more ergonomic head position on the rifle's stock.

V. BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily apparent with reference to the following detailed description of the invention, when taken in conjunction with the appended claims and accompanying drawings, wherein:

FIG. 1(a) is a schematic showing the need to change eye position when a shooter is aiming through his telescopic sight instead of using the open sights.

FIG. 1(b) is a schematic showing the need to change eye position when a shooter is using a combination sight formed by mounting a reflector sight on top a telescopic sight.

FIG. 2(a) is a schematic showing a wedge prism deviating the direction of a light beam via refraction.

FIG. 2(b) is a schematic showing an achromatic prism.

FIG. 3 is a schematic showing the preferred embodiment of the invention.

FIG. 4 is a perspective view showing a suitable housing means 15 in accordance with the preferred embodiment of the invention.

FIG. 5(a) shows how the invention can be used to improve eye position ergonomics of a hunting rifle scope.

FIG. 5(b) shows how the invention can be used to improve eye position ergonomics of a combination sight.

VI. DETAILED DESCRIPTION OF THE INVENTION A. Principle of Operation

It is well-known that a wedge prism would deviate the direction of a monochromatic beam of light by α degrees where

α=θ−ξ+sin⁻¹(√{square root over (n ²−sin² θ)}×sin ξ−sin θ cos ξ).  (1)

In the above formula α is the beam deviation angle, ξ the apex angle of the prism, θ is the incidence angle of the incoming beam and n is the index of refraction of the glass material used for making the prism. This notation is shown in FIG. 2(a).

For a wedge prism with small apex angle ξ and when the incidence angle θ is not too steep, (1) is simplified as

α≈(n−1)ξ.  (2)

When a wedge prism is used to bend the path of a collimated (parallel) polychromatic light beam, the only major abberation introduced is chromatic change in the beam deviation angle. This abberation can be corrected by using an achromatic prism. An achromatic prism is made by combining two or more prisms of different refractive index as shown in FIG. 2(b). The component prisms are so designed and placed that a ray of polychromatic light passing through the prism is deviated but not dispersed into a spectrum. Persons skilled in the art of optical engineering are familiar with the design of achromatic prisms so this topic is not discussed further in this section.

B. The Preferred Embodiment of the Invention

The preferred embodiment of the invention is illustrated in FIGS. 3 and 4. With reference to FIG. 3, the preferred embodiment of the invention is in the form of an attachment device that comprises a wedge prism 5 and a housing means 15 for detachably mounting the wedge prism 5 onto the eyepiece end of an existing telescopic gun sight.

The telescopic sight has an objective lens 1 and an eyepiece lens 3. These two lenses (together with an erecting system not shown in the drawings) provide a magnified image of the target visible to the shooter. It is well known that the light rays emerging from the eyepiece lens 3 of a telescope are nearly parallel (collimated). Therefore the wedge prism 5 does not introduce any significant abberations (e.g. coma, astigmatism, etc.) into the image produced by the telescopic sight. A small amount of color separation is introduced which, if desired, can be mitigated by using an achromatic prism as shown in FIG. 2(b).

The main effect of the wedge prism 5 is tilting the viewing axis 7 of the telescopic sight. The tilt angle is determined by the apex angle ξ of the prism and its index of refraction n. One can use equations (1) and (2) in the previous section to achieve a desired tilt angle α.

The housing means 15 is preferably constructed in the form of a simple cylindrical enclosure made of rubber or other elastic material as shown in FIG. 4. The cylindrical enclosure is designed such that it fits firmly on the eyepiece end of an existing riflescope. Alternatively, the housing means 15 may comprise an enclosure made of metal or other solid material and attachment means such as screws, threads, clamps, or magnets such that the wedge prism 5 can be mounted firmly, yet detachably, onto the rear end of a telescopic gun sight. These alternative methods of attachment are very simple and well-known hence they are not depicted in the drawings. Persons skilled in the art of opto-mechanical engineering would be familiar with designing and making a suitable housing means according to the invention.

C. How to Use the Invention

Two examples of how to use this invention are illustrated in FIGS. 5(a) and 5(b).

FIG. 5(a) shows a use case where the attachment is attached to a riflescope mounted on a rifle which also has iron sights. In this case the wedge prism 5 is positioned with its base facing downwards. This causes the riflescope's viewing axis to tilt downwards. The tilted viewing axis allows the shooter to aim at his target from the same head position he would use when aiming with the rifle's open sights. This greatly enhances aiming ergonomics and eliminates the need for a raised rifle stock (Compare FIG. 5(a) with FIG. 1(a)).

FIG. 5(b) shows a use case where the prismatic attachment is used with a riflescope mounted on an assault rifle together with a reflex sight. In this case the attachment is attached to the eyepiece end of the riflescope such that the base of the wedge prism 5 is facing upwards. This causes the riflescope's viewing axis to tilt upwards which, in turn, allows the shooter to aim from a higher head position in line with the reflex sight. This greatly enhances aiming ergonomics and eliminates the need for changing head position when switching between the reflex sight and the telescopic sight (Compare FIG. 5(b) with FIG. 1(b)).

D. Advantages

The invention is very simple and inexpensive. It can be used with most any telescopic gun sight. It is easy to use and provides enhanced ergonomics for the user.

VII. CONCLUSION, RAMIFICATIONS, AND SCOPE

The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention.

It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance presented herein, in combination with the knowledge of one of ordinary skill in the art. Thus, the scope of the invention should be determined by the appended claims and their legal equivalents, as opposed to the embodiments illustrated. 

What is claimed is:
 1. An attachment device for use with a telescopic gun sight, comprising a. a housing means, said housing means being substantially cylindrical in shape, said housing means being detachably mountable onto the eyepiece end of the telescopic gun sight, and b. a wedge prism, said wedge prism having a predetermined apex angle, said wedge prism being mounted inside said housing means such that the light rays exiting from the telescopic gun sight pass through it whereby an operator can aim through the telescopic gun sight from an offset eye position.
 2. The attachment device of claim 1 wherein said wedge prism is comprised of an achromatic prism.
 3. The attachment device of claim 1 wherein said housing means is made of rubber or similar elastic material such that it can snugly fit onto the eyepiece end of a telescopic gun sight.
 4. The attachment device of claim 1 wherein said housing means is made of aluminium or a similar light metal.
 5. The attachment device of claim 1 wherein said housing means is made of plastic or a similar synthetic material.
 6. The attachment device of claim 1 wherein said housing means further includes a clamp for attaching it onto the eyepiece end of a telescopic gun sight.
 7. The attachment device of claim 1 wherein said housing means further includes screw threads for attaching it onto the eyepiece end of a telescopic gun sight. 